A standard concrete pump has a hopper adapted to hold the viscous concrete mass and having a front wall formed with a front port at a front axis perpendicular to the front wall at the front port and a rear wall formed with two rear ports centered on respective rear axes perpendicular to the rear wall at the rear ports and generally parallel to the front axis. Respective piston pumps secured to the rear wall outside the hopper over the rear ports can draw portions of the mass out of the hopper and expel the drawn-out portions back into the hopper through the respective rear ports. An outlet conduit is connected to the front wall outside the hopper over the front port. A nonstraight distributor pipe in the hopper has a front end engaged over and aligned with the front port and a rear end engageable over and alignable with either of the rear ports. The distributor pipe can be pivoted about the front axis between one position with the rear end aligned with and engaged over one of the rear ports and with the other rear port opening into the hopper and another position with the rear end aligned with and engaged over the other rear port and with the one rear port exposed in the hopper.
The hopper is filled with concrete and the pumps are operated alternately, with the one expelling concrete into the distributor pipe while the other, whose rear port is exposed in the hopper, draws in a portion. This produces in the distributor pipe and outlet conduit connected to it a nearly continuous flow interrupted only momentarily as the distributor pipe moves between its end positions. It is possible in this manner to displace concrete which is an extremely heavy, abrasive, viscous, and corrosive material that either could not be displaced by any conventional pump or that would quickly destroy it.
Various configurations of such pumps are known, such as described for example in commonly owned U.S. application Nos. 427,180, 427,300, and 427,301 all filed Sept. 29, 1982 as well as in German patent document Nos. 1,285,319, 2,162,406, and 3,045,885 discussed in these U.S. applications. In them the distributor pipe is pivoted back and forth about the front axis by a heavy-duty double-acting hydraulic ram. A set of heavy valves is connected between a pressure source and sump on one side and the three actuators for the two pumps and distributor pipe on the other side to synchronously operate these devices. The reversal of each of the actuators is normally controlled by two position detectors, typically reed-type limit switches that operate the valve solenoids.
In order to make the machine operate more smoothly, it has been suggested on page 35ff of Hydraulische Arbeitszylinder by Hans Lang (Krauskopf Verlag, Mainz; 1964) to provide a braking or damping arrangement which is based on restricting the outflow. As a result the cylinder is braked at the end of its stroke by blocking the outflow of the hydraulic liquid with a restriction which, for instance, is controlled by the movement of the cylinder, closing as the cylinder reaches an end position. Such an arrangement continues to subject the working, that is pressurized, compartment of the actuator to full system pressure, which is usually considerable, so that the restriction heats considerably. Typically the cylinder is set so that the piston bottoms at the respective end of the cylinder in its end positions. In such an arrangement most of the system is under very high pressure at all times, leading to premature wear and considerable energy waste.
It has also been suggested to hold the distributor pipe in its end positions by pressing it forcibly against an abutment, as otherwise the mass movement as the other pump draws some of the concrete out of the hopper can pull this pipe out of alignment with the rear port it is receiving concrete from. Any misalignment between the rear end of the pipe and the rear port it is covering leads to substantial wear and rapid equipment failure. Thus the one compartment of the cylinder that displaces the pipe remains fully pressurized while the pipe is receiving concrete and the other compartment is connected directly to the sump. As mentioned above, such constant pressurization is hard on all the related equipment.